A simplest control system of a legged mobile robot, in particular a biped walking robot comprises a desired motion pattern generator and a joint drive controller. The desired motion pattern generator generates at least a desired motion pattern. Normally, the desired motion pattern is generated in such a manner that a ZMP trajectory, obtained therefrom by conducting a dynamics calculation thereon, i.e., by solving the Euler-Newton equation, tracks a predetermined desired trajectory. The joint drive controller controls to drive the robot joints such that the joint displacement tracks the joint displacement command of the respective joints generated by the generator.
Here, the ZMP (Zero Moment Point) is used in this specification to indicate a floor point at which the moment components, except for the vertical component, of the resultant force (of the inertial force, the gravity generated by the robot motion) are both zero.
In such a control system, if the generator anticipates the floor flat and generates a gait suitable therefor, but the floor has, in fact, a slant as illustrated in FIG. 40. If the robot foreleg foot lands on the unexpected slant at the beginning of the two-leg supporting period, the foot generates an excessive floor reaction force greater than that anticipated, causing the robot to tilt. In order to solve this problem, the applicant proposed, in Japanese Laid-Open Patent Application Hei 5 (1993)--305,586, a control system for a biped walking robot of this kind.
In the proposed system, the robot body inclination is detected to determine a moment-restoring-demand value, while the moment component of the actual total floor reaction force about a desired total floor reaction force central point (the central point of the total floor reaction force; a desired ZMP) is detected, and the robot feet are controlled to move up and down such that the detected moment component of the actual total floor reaction force becomes equal to the moment-restoring-demand value. The moment component of the actual total floor reaction force is a moment which the resultant force of all of the foot floor reaction forces generates about the desired total floor reaction force central point (i.e., the desired ZMP).
Taking as an example the case illustrated in FIG. 40 in which the floor has the unexpected slant, the control system proposed earlier by the applicant will be explained. (The control of the system will be hereinafter referred to as "two-leg-compliance control".) For ease of understanding, each foot is assigned with a reference numeral, as illustrated in the figure. It is assumed here that, although the gait generator anticipates the floor flat and generates a gait suitable therefor, but the floor has, in fact, the slant as illustrated in FIG. 40, and hence, since the robot foreleg foot lands on the unexpected slant at the beginning of the two-leg supporting period, the foot generates an excessive floor reaction force greater than that anticipated. It is also assumed that the robot still keeps, at that instant, a desirable posture (i.e., the body inclination zero).
In the proposed control system, the actual moment of total floor reaction force about the desired total floor reaction force central point (i.e., the desired ZMP) is detected. At that situation, since the vertical component of the floor reaction force of the 1st foot is excessive, the actual moment of total floor reaction force acts in the direction to tilt the robot backward.
In the proposed control system, in order to decrease the moment to zero, a virtual floor A-A' is supposed, as illustrated in FIG. 41, and the virtual plane is supposed to be rotated by an angle .DELTA..theta. about the desired total floor reaction force central point (desired ZMP) and each foot is supposed to be on the virtual plain such that the feet are moved to the positions on the virtual floor.
With this, the vertical component of the 1st foot floor reaction force decreases, while the vertical component of the 2nd foot floor reaction force increases. As a result, the actual moment of total floor reaction force about the desired total floor reaction force central point (desired ZMP) becomes almost zero. Thus, even if the floor has an unexpected slant, this two-leg-compliance control can ensure that the robot continues walking without tipping over.
However, the proposed technique can not control the actual floor reaction force acting on each foot during the two-leg supporting period, if the floor has an unexpected local slant or bump, the robot is likely to spin or may tip over due to drastic posture change.
To be more specific, as illustrated in FIG. 42, if there exists an unexpected protrusion or step (level difference) on the floor at a position at which the robot 1st foot toe is scheduled to land in the two-leg supporting period, since the 1st foot toe is controlled to be driven downward in the two-leg supporting period, the 1st foot toe will stomp the projection, causing the vertical component of the 1st foot floor reaction force to grow drastically. As a result, this suddenly generates the actual moment of total floor reaction force about the desired total floor reaction force central point (desired ZMP). The two-leg-compliance control would sometimes be late in restoring the posture and at worst, the robot turns over.
Even if the robot did not turn over during the two-leg supporting period, when the second foot is lifted, although the desired total floor reaction force central point (desired ZMP) is set at the heel of the 1st foot, since the 1st foot heel is not on the floor, the actual total floor reaction force central point shifts to its toe. The generated actual moment of total floor reaction force about the desired total floor reaction force central point (desired ZMP) will tilt the robot backward and cause the robot to turn over.
It could be stated from the above that the two-leg-compliance control can effectively cope with an unexpected slant or undulation extending over a relatively long distance, but can not cope with an unexpected local slant or level difference existing at a position at which the robot foot will land.
Aside from the aforesaid two-leg-compliance control system, the applicant proposed, in Japanese Laid-Open Patent Application No. Hei 5 (1993)--305,584, etc., another control system which has a foot-landing-impact-absorber made of a material such as rubber with a springy property. In this control system, actual moment of foot floor reaction force acting about the ankle of each robot foot is detected and an ankle compliance control to rotate the foot ankle such that the detected moment becomes zero, is effected.
In order to solve the problem mentioned above, it is therefore possible to combine this control disclosed in Japanese Laid-Open Patent Application No. Hei 5 (1993)--305,584 (hereinafter referred to as "ankle-compliance control") to the two-leg-compliance control.
If the two kinds of control are used, it will be possible to rotate the 1st ankle in the direction in which the unexpected moment of the 1st foot floor reaction force is canceled, as illustrated in FIG. 43, such that the 1st foot heel lands on the floor. Accordingly, the robot will not turn over when the phase shifts to the succeeding one-leg supporting period.
However, if the two-leg-compliance control and the ankle-compliance control are simply combined to be used, the two kinds of control interferes with each other and causes either or both of the total floor reaction force and the foot floor reaction force to deviate from desired values or control oscillates.
An object of the present invention is to solve the drawbacks and to provide a control system for a legged mobile robot which can ensure to control the actual floor reaction force acting on the robot easily and properly, while preventing the problem of interference from occurring.
A second object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot properly, even when walking on the floor having not only a slant or undulation extending over a relatively long distance, but also an unexpected local slant or level difference, without being affected thereby.
A third object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot properly such that a posture stabilization control of a legged mobile robot is facilitated.
A fourth object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot properly such that the foot-landing impact is decreased.
Moreover, the biped walking robot walks with its free leg swinging forward. This yields the moment of inertia force about the vertical axis of the robot, which causes the robot body to rotatively-vibrate about that axis. With this, the vertical component of the actual moment of floor reaction force acting on each foot oscillates. If the magnitude of oscillation becomes excessive, the actual moment of foot floor reaction force peaks beyond the friction limit, causing the robot foot to slip and the robot to spin. If the robots spins greatly, it may sometimes lose posture stability and turn over. Accordingly, it is desirable to conduct, in addition to the aforesaid control, a control to decrease such an oscillation.
A fifth object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot properly such that the oscillation of the vertical component of the moment of floor reaction force is decreased.
A sixth object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot properly such that the contactability of robot foot with the floor is enhanced so as to prevent the aforesaid slippage or spinning that can occur during walking from happening.
A seventh object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot properly such that the load to be exerted on actuators of the robot is decreased.